Molecular interactions leading to biochemical processes, especially the interaction of small molecules with their receptors, i.e., enzymes, antibodies, transport systems, hormone receptors, are under extensive investigation. A promising approach to identifying complex biological receptors is that of photoaffinity labeling. The research proposed here is directed towards studying the mechanism of photoaffinity labeling using time resolved electronic and vibrational laser spectroscopy. The aim of these experiments is three fold: 1) to determine the energetics, kinetics and reaction mechanisms of such important reactive intermediates as carbenes and nitrenes. 2) To understand mechanisms for labeling of active sites of protein molecules. 3) To translate the mechanistic and kinetic data into criteria for devising versatile and specific labeling reagents.